1. Field of the Invention
The present invention relates to a fluid droplet ejecting system and more particularly to an ink-jet recording apparatus which comprises a piezoelectric element and a nozzle having a liquid chamber and which is so constructed that the piezoelectric element is driven by pulse voltages to apply pressure to the liquid chamber to thereby eject the liquid in the form of fluid droplets.
2. Description of the Prior Art
Existing ink jet recording devices of the prior art are first summarized below:
If to a nozzle-having liquid chamber is applied pressure so that, for example, the inside volume of the liquid chamber contracts, then the liquid inside the liquid chamber becomes compressed thereby to be ejected from the nozzle of the liquid chamber. If the pressure is applied suddenly to the liquid chamber, then the liquid also is compressed suddenly, causing the liquid ejected from the nozzle to become fine fluid droplets.
If an ink is used as the liquid inside the liquid chamber and a recording medium such as a sheet of recording paper is provided in front of the nozzle and when the above operation is effected in accordance with recording signals (pulse voltages), then the ink droplets ejected from the nozzle strike the recording paper, thus forming ink dots on the recording paper. In this operation if, for example, the recording paper is moved in the vertical direction, while the nozzle is moved in the lateral direction, then the dot recording can be made to form desired character or letter patterns over the whole area of the recording paper.
Such the above-described recording apparatus, as the on-demand-type ink-jet recording apparatus, is already on the market.
The on-demand-type ink-jet recording apparatus is not one recording dots on a recording sheet by the impact of a wire as in wire-dot-type printers but one in which the minute ink droplets jetted from the nozzle strike a recording sheet to thereby form dots thereon, so that the recording can be carried out very quietly. As means for applying pressure to the ink liquid if, for example, a piezoelectric element is used which has a nature of being strained by the application of voltages thereto, the number of driving means necessary for the recording can be reduced, allowing to make the apparatus compact and to shorten the recording operation period.
Namely, the on-demand-type ink-jet recording apparatus is capable of making recording operation much more noiselessly and faster than does the wire-dot-type printer and, besides, can be of a compact type. Further, the apparatus allows the use of a plurality of inks different in color to make superposed printings at same points on a recording sheet, thereby enabling to produce a multicolor recording comprising not only the colors of individual inks themselves but also a variety of mixed colors.
In the on-demand-type ink-jet recording apparatus, in order to record a high-density and high-resolution information on a recording sheet, it is necessary to minimize the size of each of the dots to be recorded on a recording sheet, and for this purpose, the size of each of the droplets to be ejected from the nozzle must be minimized.
Further, in recording an image or the like on a recording sheet, multistage change in the density is necessary. The multistage change in the density can be carried out by changing the number of dots per unit area on a recording sheet. For example, a high-density recording can be obtained by increasing the number of dots, while a low-density recording can be obtained by reducing the number of dots. However, this method has its limit to the representation of halftone gradation.
In order to change the density by multiple stages it is necessary to change not only the number of dots per unit area but also dot size.
That is, in the on-demand-type ink-jet recording apparatus, in order to record a high-density, high-resolution and multistage-density information on a recording sheet, it is desirable that the size of the droplet ejected from the nozzle be minimized.
Minimization of the size of the droplet to be ejected from the nozzle is considered able to be attained by minimizing the diameter of the orifice of the nozzle, but this tends to cause the nozzle to be clogged and to increase the friction of an ink with the nozzle orifice, so that the ink is hardly ejected from the nozzle. There is naturally a limit to minimizing the diameter of the nozzle orifice with relation to the fluidity of the ink liquid that passes through the nozzle.
For increasing the density of information to be recorded on a recording sheet there is also a method which utilizes satelite droplets, much smaller fluid droplets, that are formed secondarily behind the ink droplets when ejected from the nozzle. Since the main droplets and satelite droplets that are ejected from the nozzle are ejected in the same direction, their striking points, if no manipulation is applied thereto, are the same. In order to minimize the size of each of the dots to be recorded on a recording sheet, satelight droplets alone must be used with manipulation to prevent the main droplets from arriving at the recording sheet. For this reason, means for charging and deflecting the main droplets and a device for recovering the unused main droplets are required, so that the recording apparatus needs to be of a large size and becomes expensive.
In an effort to minimize the size of each of the droplets to be ejected from the nozzle to minimize the size of each of the dots to be recorded on a recording sheet there Was devised a device which lowers pulse voltages to be applied to an electric-mechanical converter to thereby lower the pressure to be applied to the ink liquid inside the ink chamber.
However, it has been found out that even this device can hardly minimize the size of the ink droplet. The ink chamber, electric-mechanical converter, and the like, which constitute the ink-ejecting apparatus, have their own intrinsic oscillation frequencies. If the oscillation frequency produced by the pulse voltage to be applied to the electric-mechanical converter is not coincident with the foregoing intrinsic oscillation frequency and does not resonate, then ink droplets of a given uniform size are ejected efficiently from the nozzle by the applied pressure, but if the oscillation frequency produced by the pulse voltage to be applied to the electric-mechanical converter is identical in the frequency component with the resonant oscillation frequency, ink droplets are not sufficiently ejected from the nozzle.
As has been described, these existing techniques are unable to readily have any desired small size droplets ejected from the nozzle.
As a result of our investigation it has now been found out that even where the same pulse voltage is applied to the electric-mechanical converter, the position of the tip end of the ink liquid inside the nozzle at the time when the pulse voltage is applied has relation to the droplet size.
That is, even when the same pulse voltage is applied to the electric-mechanical converter, the droplets ejected from nozzle 4 differs in the size between when the tip end of ink liquid 3 inside nozzle 4 of ink chamber 2 comes up to the orifice of nozzle 4 as shown in FIG. 1(a) and when the tip end of ink liquid 3 inside nozzle 4 is at a certain distance from the orifice of nozzle 4 as shown in FIG. 1(b).
If the diameter of an ink droplet is taken for the axis of ordinate and the distance l between the orifice of nozzle 4 and the tip position of ink liquid 3 is taken for the axis of abscissa, then the droplet size changes as shown in FIG. 2 even when the same pulse voltage is applied.
In an ink-jet recording head as shown in FIG. 3(a), when a pulse voltage as shown in FIG. 4(a) is applied to the piezoelectric element 1 provided on the wall of ink chamber 2, if the wall of ink chamber 2 is strained to be bent as, e.g., indicated with the assumed lines, every time when the pulse voltage is applied, then ink liquid 3 inside chamber 2 becomes compressed to thereby eject ink liquid 3 in the form of droplets from nozzle 4.
If to the foregoing piezoelectric element 1 of the ink-jet head is applied as shown in FIG. 4(b) a pulse voltage of the opposite polarity to that of FIG. 4(a), then this time the wall of ink chamber 2 having thereon piezoelectic element 1 is strained to be bent outward to increase the volume of chamber 2 as shown in FIG. 3(b). And when the pulse voltage applied to piezoelectric element 1 is stopped to thereby return the wall having thereon the piezoelectric element to the original position, the pressure caused by the return is applied to ink liquid 3 to thereby eject ink droplets from nozzle 4.
The bending direction of piezoelectric element 1, depending on the polarity of the voltage applied, becomes opposite, but any of both polarities can put pressure upon the ink liquid inside the chamber to eject ink droplets 5 from nozzle 4.
In comparison of FIG. 3(a) with FIG. 3(b), the droplet size when ejected from nozzle 4 of the ink-jet printing head as shown in FIG. 3(b) becomes smaller for the following reason.
In the case of FIG. 3(b), when a pulse voltage is applied to piezoelectric element 1, the element is strained so as to increase the volume of chamber 2, so that the pressure of the liquid inside chamber 2 is reduced. The reduction of the pressure inside chamber 2 causes ink liquid 3 inside nozzle 4 to be drawn back toward the chamber side, so that the distance l between the tip of nozzle 4 and the tip position of ink liquid 3 becomes elongated as shown in FIG. 1(b). Accordingly, the ink-jet printing head shown in FIG. 3(b) is considered to eject smaller droplets from the nozzle than those in the case of FIG. 3(a).
On the other hand, the piezoelectric element, during its manufacture, is subjected to a voltage in a certain direction in order to align the polarization direction. The piezoelectric element is made of a ferrodielectric material. When the same polar voltage as that in the polarization treatment is applied, the polarization direction is consolidated, but when the opposite polar voltage is applied repeatedly to the piezoelectric element, the polarization direction becomes disturbed, so that even when a pulse voltage is applied, the degree of the strain becomes reduced, no expected pressure is put on the liquid, and therefore no droplets are ejected from the nozzle. As far as the piezoelectric element is made of a ferrodielectric material, such the phenomenon is unavoidable. Up to now, in the case where a voltage of the opposite polarity to that in the polarization treatment was applied to the piezoelectric element, when the degree of the strain became lowered, the piezoelectric element had to be replaced with a new one.